TW> The CEL light is more of a Engine Computer Check system. The smog device
TW> monitiring is jsut an add on. My now gone Chev S-10 even had the
TW> automatic transmission monitored by the CEL system

Yes, it monitors all types of sensor related data.  

I have one of those code readers, but also have the cable/software laptop
setup that can give me way more information.  In addition to that, I also
have an anti-lock brake diagnostics that can plug into the same OBD
connection.

I have used the anti-lock reader many times to figure out what was causing
my several anti-lock break errors that I have seen over the years.  It is
always a "speed sensor", but the trick is knowing which wheel/hub
has the issue.  It is a complete guessing game without this tool.  

It will tell you which one is bad, and then you have to replace the
wheel/hub since it is an enclosed unit.

- Mark

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